A display panel such as a transmissive or transflective liquid crystal display (LCD) panel requires a back-lighting source for illumination. Light-emitting devices (LEDs) are commonly used in such a back-lighting source. In particular, LEDs in red, green and blue colors are used to provide a back-light source in “white” color. To illuminate a large LCD panel, many strips of LEDs in different colors are used in a back-light source. The LED strips in different colors are driven by different LED drivers.
In order to control the “whiteness” of the back-lighting source, three sets of sensors are typically used to sense the color brightness level in red, green and blue separately. The sensed color levels are conveyed to a processing means in a feedback control circuit so as to allow the processing means to adjust the color brightness levels through the LED drivers. For example, Muthu et al. (U.S. Patent Application Publication No. 2003/0230991) discloses a feedback circuit wherein photodiodes with color filters are used to send feedback to a microprocessor via a signal conditioning circuit. The microprocessor is programmed to provide signals that control currents from the LED drivers. These signals can take the form of amplitude modulation or pulse width modulation (PWM) so as to change the currents. Chang (U.S. Patent Application Publication No. 2003/0011832) discloses a method for controlling the brightness of the red, green and blue LEDs in a white light source based on the color chromaticity coordinates of the LEDs. Schuurmans (U.S. Patent Application Publication No. 2003/0076056) discloses a color sensing method wherein three sets of color filtered photodiodes and one set of unfiltered photodiodes are used to measure the ratio of the filtered to unfiltered brightness in each color so as to estimate the tristimulus values or the color point of the light source. Based on the difference between the estimated color point and the target color point, a control circuit modifies the driving currents to the color LEDs.
In prior art, the LED driving currents are modified by using pulse-width modulation (PWM) to change the duty cycle of each LED strip while maintaining the same frequency. As illustrated in FIG. 1, the back-lighting source control system 1 comprises a back-lighting source 20 and an LED driving system 10. The back-lighting source 20 contains a plurality of LED strips 24, 26, . . . of different color LEDs driven by a plurality of drivers 14, 16, . . . A sensor 30 is used to sense the color components of the LED strips. Electrical signals indicative of the sensed brightness from the sensor are conveyed to a controller 12 in the LED driving system 10. Upon measuring the color levels in the back-lighting source, the controller adjusts the brightness in the LED strips by changing the duty cycle of the LED drivers 14, 16, . . . using PWM. As illustrated in FIG. 1, although the duty cycle in LED strip #1 may be different from LED strip #2, the driving frequency in the LED strips is the same. For a small back-lighting source, this prior art method may be adequate in adjusting the overall brightness and “whiteness” of the light output. However, a single sensor may not be sufficient in monitoring the uniformity of the light output throughout the back-lighting source. Thus, it may be necessary to place two or more sensors at different sites to sense the color levels at different locations. As shown in FIG. 1, a second sensor 30′ is also used to sense the color levels at a different place for improving the output uniformity of the back-lighting source.
The use of multiple sensors increases the cost and the complexity of the monitoring system. It is thus advantageous and desirable to provide a more cost-effective method and a system for color level adjustment and control.